There is a 5000 character limit in the job overview field for the candidate portal.
The character count will include HTML tags. So the solution would be to click on the 'Source' button on the bottom row of the 'WYSIWYG' editor on the Job Description field, and paste the ad content into there. This will strip all HTML from the content.
Then click the 'Source' button again, and format your ad using the WYSIWYG editor.